Melting furnace including wire-discharge ion plasma electron emitter

ABSTRACT

An apparatus for melting an electrically conductive metallic material includes a vacuum chamber and a hearth disposed in the vacuum chamber. At least one wire-discharge ion plasma electron emitter is disposed in or adjacent the vacuum chamber and is positioned to direct a wide-area field of electrons into the vacuum chamber, wherein the wide-area electron field has sufficient energy to heat the electrically conductive metallic material to its melting temperature. The apparatus may further include at least one of a mold and an atomizing apparatus which is in communication with the vacuum chamber and is positioned to receive molten material from the hearth.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §120 as acontinuation of co-pending U.S. patent application Ser. No. 12/055,415,filed Mar. 26, 2008, which in turn claims priority under 35 U.S.C.§119(e) to U.S. Provisional Patent Application Ser. No. 60/909,118,filed Mar. 30, 2007.

BACKGROUND OF THE TECHNOLOGY

1. Field of Technology

The present disclosure relates to equipment and techniques for meltingmetals and metallic alloys (hereinafter “alloys”). The presentdisclosure more specifically relates to equipment and techniquesutilizing electrons to melt metals and alloys and/or the materials whilein a molten state.

2. Description of the Background of the Technology

The alloy melting process involves preparing a charge of suitablematerials and then melting the charge. The molten charge or “melt” maythen be refined and/or treated to modify melt chemistry, removeundesirable components from the melt, and/or affect the microstructureof articles cast from the melt. Melting furnaces are powered by eitherelectricity or the combustion of fossil fuels, and selection of asuitable apparatus is largely influenced by the relative costs andapplicable environmental regulations, as well as by the identity of thematerial being prepared. A variety of melting techniques and apparatusare available today. General classes of melting techniques include, forexample, induction melting (including vacuum induction melting), arcmelting (including vacuum arc skull melting), crucible melting, andelectron beam melting.

Electron beam melting typically involves utilizing thermo-ionic electronbeam guns to generate high energy substantially linear streams ofelectrons which are used to heat the target materials. Thermo-ionicelectron beam guns operate by passing current to a filament, therebyheating the filament to high temperature and “boiling” electrons awayfrom the filament. The electrons generated from the filament are thenfocused and accelerated toward the target in the form of a very narrow,substantially linear electron beam. A type of ion plasma electron beamgun also has been used for preparing alloy melts. Specifically, a “glowdischarge” electron beam gun described in V. A. Chernov, “PowerfulHigh-Voltage Glow Discharge Electron Gun and Power Unit on Its Base”,1994 Intern. Conf. on Electron Beam Melting (Reno, Nev.), pp. 259-267,has been incorporated in certain melting furnaces available fromAntares, Kiev, Ukraine. Such devices operate by producing a cold plasmaincluding cations which bombard a cathode and produce electrons that arefocused to form a substantially linear electron beam.

The substantially linear electron beams produced by the foregoing typesof electron beam guns are directed into the evacuated melting chamber ofan electron beam melting furnace and impinged on the materials to bemelted and/or maintained in a molten state. The conduction of electronsthrough the electrically conductive materials quickly heats them to atemperature in excess of the particular melting temperature. Given thehigh energy of the substantially linear electron beams, which can be,for example, about 100 kW/cm², linear electron beam guns are very hightemperature heat sources and are readily able to exceed the melting and,in some cases, the vaporization temperatures of the materials on whichthe substantially linear beams impinge. Using magnetic deflection orsimilar directional means, the substantially linear electron beams arerastered at high frequency across the target materials within themelting chamber, allowing the beam to be directed across a wide area andacross targets having multiple and complex shapes.

Because electron beam melting is a surface heating method, it typicallyproduces only a shallow molten pool, which can be advantageous in termsof limiting porosity and segregation in the cast ingot. Because thesuperheated metal pool produced by the electron beam is disposed withinthe high vacuum environment of the furnace melting chamber, thetechnique also beneficially tends to degas the molten material. Also,undesirable metallic and non-metallic constituents within the alloyhaving relatively high vapor pressures can be selectively evaporated inthe melting chamber, thereby improving alloy purity. On the other hand,one must account for the evaporation of desirable constituents producedby the highly-focused substantially linear electron beam. Undesirableevaporation must be factored into production and can significantlycomplicate alloy production when using electron beam melting furnaces.

Various melting and refining methods involve the electron beam meltingof feed stocks using thermo-ionic electron guns. Drip melting is aclassic method used in thermo-ionic electron beam gun melting furnacefor processing refractory metals such as, for example, tantalum andniobium. Raw material in the form of a bar is typically fed into thefurnace chamber and a linear electron beam focused on the bar drip-meltsthe material directly into a static or withdrawal mold. When casting ina withdrawal mold, the liquid pool level is maintained on the top of thegrowing ingot by withdrawing the ingot bottom. The feed material isrefined as a result of the degassing and selective evaporation phenomenadescribed above.

The electron beam cold hearth melting technique is commonly used in theprocessing and recycling of reactive metals and alloys. The feedstock isdrip melted by impinging a substantially linear electron beam on an endof a feedstock bar. The melted feedstock drips into an end region of awater-cooled copper hearth, forming a protective skull. As the moltenmaterial collects in the hearth, it overflows and falls by gravity intoa withdrawal mold or other casting device. During the molten material'sdwell time within the hearth, substantially linear electron beams arequickly rastered across the surface of the material, retaining it in amolten form. This also has the effects of degassing and refining themolten material through evaporation of high vapor pressure components.The hearth also may be sized to promote gravity separation betweenhigh-density and low-density solid inclusions, in which case oxide andother relatively low-density inclusions remain in the molten metal for atime sufficient to allow dissolution while high density particles sinkto the bottom and become trapped in the skull.

Given the various benefits of conventional electron beam meltingtechniques, it would be advantageous to further improve this technology.

SUMMARY

According to one non-limiting aspect of the present disclosure, anembodiment of an apparatus for melting an electrically conductivemetallic material is described. The apparatus includes a vacuum chamber,a hearth disposed in the vacuum chamber, and at least one wire-dischargeion plasma electron emitter within or adjacent to the vacuum chamber.The wire-discharge ion plasma electron emitter is positioned to direct awide-area electron field into the chamber, wherein the electron fieldhas energy sufficient to heat the electrically conductive metallicmaterial to its melting temperature. A mold or other casting oratomizing device is in communication with the chamber and is positionedto receive molten material from the hearth. Certain non-limitingembodiments of the melting apparatus may be operated at chamberpressures greater than chamber pressures of conventional electron beammelting furnaces so as to reduce or eliminate the vaporization in thechamber of volatile elements from the molten material.

According to another non-limiting aspect of the present disclosure, anelectron beam cold hearth melting furnace is described including avacuum chamber and a hearth disposed in the vacuum chamber, the hearthincluding a molten material holding region. The furnace further includesat least one wire-discharge ion plasma electron emitter that is disposedin or is adjacent to the vacuum chamber. The hearth and the at least onewire-discharge ion plasma electron emitter are disposed so that awide-area electron field emitted by the wire-discharge ion plasmaelectron emitter at least partially impinges on the molten materialholding region and any material disposed within the region. A withdrawalmold is in communication with the chamber and is positioned to receivemolten material from the hearth. At least one material feeder is incommunication with the chamber and is adapted to introduce electricallyconductive feed material into the vacuum chamber in a position over aregion of the hearth Certain non-limiting embodiments of the electronbeam cold hearth melting furnace may be operated at vacuum chamberpressures greater than chamber pressures of conventional electron beammelting furnaces so as to reduce or eliminate the vaporization in thevacuum chamber of volatile elements from the molten material.

According to yet another non-limiting aspect of the present disclosure,a method of processing a material is provided. The method includesintroducing a material including at least one of an electricallyconductive metal and metallic alloy into a melting chamber that ismaintained at a low pressure relative to atmospheric pressure. Thematerial is subjected to a wide-area electron field within the chamberso as to heat the material to a temperature above its meltingtemperature. The wide-area electron field is at least partiallygenerated by at least one wire-discharge ion plasma electron emitter.Optionally, a casting or a powder is formed from the material subsequentto or simultaneous with subjecting the material to the electron field.In certain non-limiting embodiments of the method the pressure withinthe melting chamber is greater than the melting chamber pressures ofconventional electron beam melting furnaces so as to reduce or eliminatethe vaporization in the melting chamber of volatile elements from themolten material within the melting chamber.

According to a further aspect of the present disclosure a method ofprocessing a material is provided including introducing at least oneelectrically conductive material selected from titanium, titaniumalloys, tungsten, niobium, tantalum, platinum, palladium, zirconium,iridium, nickel, nickel base alloys iron, iron base alloys, cobalt, andcobalt base alloys into a chamber maintained at a low pressure relativeto atmospheric pressure. The material is subjected to a wide-areaelectron field within the chamber to heat the material to a temperatureabove a melting temperature of the material, wherein the wide-areaelectron field is generated by a wire-discharge ion plasma electronemitter. Optionally, at least one alloying additive is added to thematerial within the chamber. A casting or a powder is formed from thematerial either subsequent to or simultaneous with subjecting thematerial to the electron field. In certain non-limiting embodiments ofthe processing method the pressure within the chamber is greater thanthe melting chamber pressures of conventional electron beam meltingfurnaces so as to reduce or eliminate the vaporization in the chamber ofvolatile elements from the molten material within the melting chamber.

According to yet a further aspect of the present disclosure a method isprovided for decreasing or eliminating undesirable evaporation ofvolatile elements from a molten material during heating of the materialin a vacuum chamber of an electron beam melting furnace. The methodincludes maintaining the pressure within the vacuum chamber at at least40 μm Hg (5.3 Pa) during at least a portion of the time for which thematerial is being heated in the vacuum chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the apparatus and methods described hereinmay be better understood by reference to the accompanying drawings inwhich:

FIG. 1 is a schematic illustration in cross-section of an embodiment ofa conventional thermo-ionic electron beam gun melting furnace;

FIG. 2 is a simplified depiction of certain components of an embodimentof a wire-discharge ion plasma electron emitter.

FIG. 3 is a schematic illustration in cross-section of one non-limitingembodiment of an electron beam cold hearth melting furnace includingmultiple wire-discharge ion plasma electron emitters according to thepresent disclosure;

FIG. 4 is a schematic illustration of one non-limiting embodiment of awire-discharge ion plasma electron emitter;

FIG. 5 is a schematic illustration of one non-limiting embodimentaccording to the present disclosure of an electron beam melting furnaceincluding a wire-discharge ion plasma electron emitter as an electronsource;

FIG. 6 is a perspective view, partly in section, of one non-limitingembodiment of a wire-discharge ion plasma electron emitter that may beadapted for use in an electron beam melting furnace according to thepresent disclosure;

FIG. 7 is a diagram illustrating operation of the wire-discharge ionplasma electron emitter illustrated in FIG. 6; and

FIG. 8 is a schematic illustration in cross-section of one embodiment ofan electron beam cold hearth melting furnace according to the presentdisclosure.

The reader will appreciate the foregoing details, as well as others,upon considering the following detailed description of certainnon-limiting embodiments of apparatus and methods according to thepresent disclosure. The reader also may comprehend certain of suchadditional details upon carrying out or using the apparatus and methodsdescribed herein.

DETAILED DESCRIPTION OF CERTAIN NON-LIMITING EMBODIMENTS

In the present description of non-limiting embodiments and in theclaims, other than in the operating examples or where otherwiseindicated, all numbers expressing quantities or characteristics ofingredients and products, processing conditions, and the like are to beunderstood as being modified in all instances by the term “about”.Accordingly, unless indicated to the contrary, any numerical parametersset forth in the following description and the attached claims areapproximations that may vary depending upon the desired properties oneseeks to obtain in the apparatus and methods according to the presentdisclosure. At the very least, and not as an attempt to limit theapplication of the doctrine of equivalents to the scope of the claims,each numerical parameter should at least be construed in light of thenumber of reported significant digits and by applying ordinary roundingtechniques.

Any patent, publication, or other disclosure material, in whole or inpart, that is said to be incorporated by reference herein isincorporated herein only to the extent that the incorporated materialdoes not conflict with existing definitions, statements, or otherdisclosure material set forth in this disclosure. As such, and to theextent necessary, the disclosure as set forth herein supersedes anyconflicting material incorporated herein by reference. Any material, orportion thereof, that is said to be incorporated by reference herein,but which conflicts with existing definitions, statements, or otherdisclosure material set forth herein is only incorporated to the extentthat no conflict arises between that incorporated material and theexisting disclosure material.

The present disclosure, in part, is directed to an improved design foran electron beam furnace for melting metals and metallic alloys and/orfor maintaining the materials in a molten state for use in preparingmetallic castings or powders. A conventional thermo-ionic electron beamgun melting furnace is schematically illustrated in FIG. 1. Furnace 110includes vacuum chamber 114 surrounded by chamber wall 115. Multiplethermo-ionic electron beam guns 116 are positioned outside and adjacentchamber 114 and direct discrete linear electron beams 118 into chamber114. Feed material in the forms of metallic bar 120 and alloying powder122 are introduced into chamber 114 by a conventional bar feeder 115 anda conventional particle or granule feeder 117, respectively. The linearelectron beam 118 of one of the electron beam guns 116 impinges on andmelts an end of bar 120, and the resulting molten alloy 124 falls intowater-cooled copper refining hearth 126 (a “cold hearth”) within chamber114. The thermo-ionic electron beam guns 116 are of a conventionaldesign and generate electrons by heating a suitable filament material.The guns 116 focus the generated electrons to a point, and the electronsare projected from the guns 116 in the form of a tightly focused,substantially linear beam. Thus, the electrons projected from the guns116 essentially impinge on the target as a point source. The heating ofthe target by the point source of electrons is facilitated by rasteringthe linear electron beams 118 across at least a portion of the targets,similar to the manner of rastering electrons across the phosphor screenof a cathode ray television tube. Rastering the substantially linearelectron beam 118 of a thermo-ionic electron beam gun 116 across the endregion of bar 120, for example, melts the bar 120.

With further reference to FIG. 1, the molten alloy 124 deposited inhearth 126 is maintained in a molten state by rastering certain of thesubstantially linear electron beams 118 across the surface of the moltenalloy 124 in a predetermined and programmed pattern. Powdered orgranulated alloying materials 122 introduced into the molten alloy 124by feeder 117 are incorporated into the molten material. Molten alloy124 advances across the hearth 126 and drops from the hearth by gravityinto a copper withdrawal mold 130. Withdrawal mold 130 includes atranslatable base 134 so as to accommodate the length of the growingingot 132. Molten alloy 124 initially collects in withdrawal mold 130 asmolten pool 131, and progressively solidifies into ingot 132.Impingement of electrons onto molten pool 131 by means of rastering oneor more of the substantially linear electron beams 118 across the pool'ssurface advantageously maintains regions of the pool 131, particularlyat the pool edges, in a molten state.

In furnaces utilizing one or more substantially linear electron beams toheat material in the furnace chamber, such as a conventionalthermo-ionic electron beam gun melting furnace, alloys includingvolatile elements, i.e., elements with relatively high vapor pressure atthe furnace melting temperatures, tend to boil off from the molten pooland condense on the relatively cold walls of the furnace chamber.(Common alloying elements having relatively high vapor pressures attemperatures commonly achieved by electron beam melting include, forexample, aluminum and chromium.) The substantially linear electron beammelting technique is particularly conducive to volatilization, which isa significant disadvantage of conventional electron beam furnaces whenalloying, as opposed to refining or purifying, for at least two reasons.First, the overall and localized chemical composition of the alloybecomes difficult to control during melting due to unavoidable losses ofhighly volatile elements from the molten pool. Second, the condensate ofthe vaporized elements tends to build up on the furnace walls over timeand can drop back into the melt, thereby contaminating the melt withinclusions and producing localized variations in melt chemistry.

Without intending to be bound by any particular theory, the inventorsbelieve that the foregoing disadvantages of conventional electron beammelting furnaces result from the action of conventional substantiallylinear electron beams on the materials processed within electron beamfurnaces. As suggested above in connection with the description of FIG.1, conventional electron beam cold hearth melting technology utilizessubstantially linear electron beams to both melt the raw materialsintroduced into the furnace and to maintain the temperature of themolten material as it flows along and over the cold hearth, and into thecasting mold. Such furnaces typically include multiple electron beamsources, wherein each source produces a substantially linear electronbeam that is essentially a point source. These “points” of intenseelectron concentration must be rastered rapidly over the areas to beheated so that the average temperature needed to melt the material andallow the molten material to adequately flow is reached throughout thetarget area. Because of the point source nature of the linear electronbeams, however, the spot at which the electron beam impinges on thealloy is heated to an extremely high temperature. This phenomenon oflocalized intense heating can be observed as visible white radiationemitted from the particular spot at which the electron beam impinges onthe solid or molten alloy within the furnace. It is believed that theintense superheating effect that occurs at these spots, along with thehigh vacuum maintained in the furnace chamber, readily evaporates therelatively volatile elements within the alloy, resulting in theexcessive evaporation of the volatile elements and concomitantcondensation on the chamber walls. As noted above, such condensationrisks contamination of the bath as the condensed material drops backinto the molten alloy, and this can result in, for example, markedcompositional heterogeneities in the cast ingots.

An improved design for an electron beam melting furnace described hereinutilizes one or more wire-discharge ion plasma electron emitters as atleast a part of the electron source in such a furnace. As used herein,the term “wire-discharge ion plasma electron emitter” refers to anapparatus that produces a relatively wide, non-linear field of electronsby impinging positively charged ions onto a cathode and therebyreleasing electrons from the cathode. The electron beam produced by awire-discharge ion plasma electron emitter is not a linear beam, butinstead is a three-dimensional field or “flood” of electrons that, whenimpinged on the target, covers a two-dimensional region that is verylarge relative to the small point covered by impinging a substantiallylinear electron beam onto the target. As such, the electron fieldproduced by wire-discharge ion plasma electron emitters is referred toherein as a “wide-area” electron field, with reference to the relativelymuch smaller point of contact produced by conventional electron gunsused in electron beam melting furnaces. Wire-discharge ion plasmaelectron emitters are known in the art (for use in unrelatedapplications) and are variously referred to as, for example, “wire ionplasma (WIP) electron” guns or emitters, “WIP electron” guns or emittersand, somewhat confusingly, as “linear electron beam emitters” (referringto the linear nature of the plasma-producing wire electrode(s) withincertain embodiments of the devices).

Wire-discharge ion plasma electron emitters are available in a varietyof designs, but all such emitters share certain fundamental designattributes. Each such emitter includes a plasma or ionization regionincluding a positive ion source in the form of an elongate wire anode toproduce plasma including cations, and a cathode that is spaced from andpositioned to intercept positive ions generated by the wire. A largenegative voltage is applied to the cathode, causing a fraction of thepositive ions in the plasma generated by the wire positive ion source tobe accelerated toward and to collide with the cathode surface such thatsecondary electrons are emitted from the cathode (the “primary”electrons being present within the plasma along with the positive ions).The secondary electrons produced from the cathode surface form anon-linear electron field that typically has the three-dimensional shapeof the positive ion plasma impacting the cathode. The secondaryelectrons are then accelerated from the vicinity of the cathode backtoward the anode, experiencing few collisions in the process of passingthrough the low-pressure gas within the emitter. By properly designingand arranging the various components of the wire-discharge ion plasmaelectron emitter, a wide field of energetic secondary electrons can beformed at the cathode and accelerated from the emitter and toward thetarget. FIG. 2 is a simplified depiction of components of awire-discharge plasma ion electron emitter, wherein a current is appliedto a thin wire anode 12 to generate plasma 14. Positive ions 16 withinplasma 14 accelerate toward and collide with negatively-charged cathode18, liberating wide-area secondary electron cloud 20, which isaccelerated in the direction of anode 12 by action of the electric fieldbetween the electrodes and toward the target.

According to one non-limiting embodiment according to the presentdisclosure, an apparatus for melting an electrically conductive metallicmaterial in the form of an electron beam melting furnace includes avacuum chamber (melting chamber) and a hearth disposed in the vacuumchamber and adapted to hold a molten material. At least onewire-discharge ion plasma electron emitter is disposed in or adjacent tothe vacuum chamber and is positioned to direct a non-linear, wide-areafield of electrons generated by the emitter into the chamber. Thewire-discharge ion plasma electron emitter produces a non-linear fieldof electrons having sufficient energy to heat the electricallyconductive metallic material to its melting temperature. A mold or othercasting or atomizing device is disposed in communication with thechamber and is positioned and adapted to receive material from thehearth. The furnace may be used to melt any material that may be meltedusing a conventional electron beam melting furnace, such as, forexample, titanium, titanium alloys, tungsten, niobium, tantalum,platinum, palladium, zirconium, iridium, nickel, nickel base alloys,iron, iron base alloys, cobalt, and cobalt base alloys.

Embodiments of an electron beam melting furnace according to the presentdisclosure may include one or more material feeders adapted to introduceelectrically conductive materials or other alloying additives into thevacuum chamber. Preferably, the feeders introduce the materials into thevacuum chamber in a position over or above at least a region of thehearth so that gravity will allow the materials, in solid or moltenform, to fall downward and into the hearth. Feeder types may include,for example, bar feeders and wire feeders, and the feeder type selectedwill depend upon the particular design requirements for the furnace. Incertain embodiments of the furnace according to the present disclosure,the material feeder and at least one of the one or more wire-dischargeion plasma electron emitters of the furnace are disposed so that theelectron field emitted by the wire-discharge ion plasma electron emitterat least partially impinges on the material introduced into the chamberby the feeder. If the material that is introduced into the vacuumchamber by the feeder is electrically conductive, then the electronfield, if of sufficient strength, will heat and melt the material.

The hearth incorporated in embodiments of a melting furnace according tothe present disclosure may be selected from the various hearth typesknown in the art. For example, the furnace may be in the nature of anelectron beam cold hearth melting furnace by incorporating a cold hearthor, more specifically, for example, a water-cooled copper cold hearth inthe vacuum chamber. As is known to those of ordinary skill, a coldhearth includes cooling means causing molten material within the hearthto freeze to the hearth surface and form a protective layer thereon. Asanother example, the hearth may be an “autogenous” hearth, which is ahearth that is plated with or fabricated from the alloy that is beingmelted in the furnace, in which case the bottom surface of the hearthalso may be water-cooled to prevent burn-through.

The particular hearth included in the vacuum chamber may include amolten material holding region, in which the molten material resides fora certain dwell time before passing to the casting or atomizing devicefluidly connected to the vacuum chamber. In certain embodiments of afurnace according to the present disclosure, the hearth and at least oneof the furnace's one or more wire-discharge ion plasma electron emittersare disposed so that the electron field emitted by the Wire-dischargeion plasma electron emitter at least partially impinges on the moltenmaterial holding region. In this way, the electron field may be appliedto maintain the material within the molten material holding region in amolten state, and the heating action of the electron field may alsoserve to degas and refine the molten material.

Certain non-limiting embodiments of a furnace according to the presentdisclosure include a mold for casting the molten material. The mold maybe any suitable mold known in the art such as, for example, a staticmold, a withdrawal mold, or a continuous casting mold. Alternatively,the furnace may include or be associated with an atomization apparatusfor producing a powdered material from the molten material.

One particular non-limiting embodiment of an electron beam meltingfurnace according to the present disclosure includes a vacuum chamberand a hearth disposed in the vacuum chamber, wherein the hearth includesa molten material holding region. The furnace further includes one ormore wire-discharge ion plasma electron emitters disposed in or adjacentthe vacuum chamber. The hearth and the at least one wire-discharge ionplasma electron emitter are disposed so that an electron field producedby the emitter at least partially impinges on the molten materialholding region. A withdrawal mold communicates with the vacuum chamberand is positioned to receive molten material from the hearth. A leastone feeder is included in the furnace and is adapted to introducematerial into the vacuum chamber in a position over at least a region ofthe hearth.

Any suitable wire-discharge ion plasma electron emitter may be used inconnection with apparatus according to the present disclosure. Suitableembodiments of wire-discharge ion plasma electron emitters are disclosedin, for example, U.S. Pat. Nos. 4,025,818; 4,642,522; 4,694,222;4,755,722; and 4,786,844, the entire disclosures of which are herebyincorporated herein by reference. Suitable emitters include thosecapable of producing a non-linear, wide-area electron field that may bedirected into the vacuum chamber of the furnace and that will heatelectrically conductive feed materials placed into the furnace chamberto the desired temperature.

In one embodiment of a wire-discharge ion plasma electron emitter, theemitter includes a plasma region and a cathode region. The plasma regionincludes at least one elongate wire anode adapted to produce a plasmaincluding positive ions. The cathode region includes a cathode which iselectrically connected to a high voltage power supply adapted tonegatively charge the cathode. In the wire-discharge ion plasma electronemitter, the electrode used to produce the plasma may be one wire ormultiple wires positioned along a length of the plasma region. At leasta portion of the cathode impacted by the positive ions is composed of amaterial suitable for generating electrons. Certain non-limitingembodiments of the cathode disposed in the cathode region of the emitteralso may include an insert, such as, for example, a molybdenum insert,having a high melting temperature and a low work function so as tofacilitate generation of electrons. The cathode and the anode arepositioned relative to one another so that the positive ions in theplasma generated by the wire anode accelerate toward and impinge on thecathode under influence of the electric field between the electrodes,liberating the wide-area field of secondary electrons from the cathode.

Certain non-limiting embodiments of the wire-discharge ion plasmaelectron emitter include at least one suitably electron transmissivewindow, such as a thin electron transmissive titanium or aluminum foil,that opens through a wall of the furnace vacuum chamber. Alternativematerials from which the electron transmissive window may be constructedinclude, for example, BN, diamond, and certain other materials composedof low atomic number elements. As discussed herein, other embodiments ofthe wire-discharge ion plasma electron emitter do not include anelectron transmissive window, in which case the plasma region of theemitter fluidly communicates with the vacuum chamber holding the moltenmaterial. In either case, the wide-area electron field derived entersthe furnace chamber and may be impinged on the material within thechamber. If an electron transmissive window does separate the interiorof the electron emitter from the vacuum chamber (as discussed furtherherein), then the electron field passes through the window as it isprojected from the electron emitter into the vacuum chamber. In certainnon-liming embodiments of a wire-discharge ion plasma electron emitter,the high voltage power supply electrically coupled to the cathode powersthe cathode to a negative voltage greater than 20,000 volts. Thenegative voltage serves the functions of accelerating the positive ionsin the plasma toward the cathode and then repelling the secondaryelectron field from the cathode and toward the anode.

An electron transmissive window is necessary if the pressure within thewire-discharge ion plasma electron emitter differs significantly fromthe pressure within the furnace chamber, in which case the foil windowserves to isolate the two adjacent regions of differing pressure. Anadvantage of wire-discharge ion plasma electron emitters relative tonon-gas containing electron emitters, such as thermo-ionic electron beamguns, is that wire-discharge ion plasma electron emitters must includegas within the plasma region to serve as the plasma source. Althoughwire-discharge ion plasma electron emitters can operate at very low gaspressures, such devices also can operate effectively at relatively highgas pressures. In contrast, conventional electron beam melting furnacescommonly operate at ultra low vacuum conditions, and in that case anelectron transmissive window would be necessary to separate the gasatmosphere within the wire-discharge ion plasma electron emitter and thenear vacuum environment within the furnace chamber. It appears, however,that volatile element evaporation within the furnace chamber may bereduced by increasing the gas pressure within the furnace chamber beyondthe ultra-low levels of conventional linear (thermo-ionic emitter)electron beam melting furnaces. Those conventional pressures levels aretypically within the range of 10⁻³ to 7.5 μm Hg (10⁻³ to 1 Pa) and donot exceed 15 μm Hg (2 Pa). Increasing pressure within the furnacechamber beyond conventional levels, i.e., to pressures exceeding 40 μmHg (5.3 Pa), or more preferably exceeding 300 μm Hg (40 Pa), increasesthe pressure at the surface of the molten material within the furnaceand thereby reduces the driving force for undesirable vaporization. Forexample, data presented in H. Duval et al., “Theoretical andExperimental Approach of the Volatilization in Vacuum Metallurgy”,suggests that there is a significant reduction in chromium vaportransport at 66.7 Pa (500 mTorr) argon relative to 4.27 Pa (35 mTorr)argon. Because wire-discharge plasma ion electron emitters alreadyrequire a partial gas pressure environment (typically of helium) to beoperational, the present inventors consider it possible that both thewire-discharge plasma ion electron emitter and the furnace chamber couldbe operated at substantially the same pressure, wherein the pressure issufficiently high to allow the electron emitter to operate and also ishigher than in conventional electron beam furnaces, thereby reducingundesirable volatilization within the furnace chamber. In such case, theelectron transmissive window may be omitted such that the gasenvironment within the emitter and the furnace chamber is substantiallythe same. Alternatively, in another embodiment of a wire-discharge ionplasma electron emitter the electrons generated by the emitter passthrough a gas impermeable window that is transparent to electrons,wherein the pressure of ionizable gas within the emitter is suitable forelectron emitter operation and the furnace chamber is operated at apressure greater than conventional pressures in electron beam furnacesand is suitable for minimizing or reducing undesirable volatilization.It will be understood that the reduction in undesirable elementalvaporization would be optimized by both utilizing one or morewire-discharge ion plasma electron emitters, which do not create pointsof intense heating, along with furnace chamber pressures greater than isconventional in electron beam furnaces.

Further discussion of possible embodiments of an electron beam meltingfurnace and possible embodiments of a wire-discharge ion plasma electronemitter useful in connection with a furnace according to the presentdisclosure are provided below.

FIG. 3 schematically illustrates one possible non-limiting embodiment ofan improved electron beam melting furnace according to the presentdisclosure. Furnace 210 includes vacuum chamber 214 at least partiallydefined by chamber wall 215. Wire-discharge ion plasma electron emitters216 are positioned outside and adjacent chamber 214. The wire-dischargeion plasma electron emitters 216 project wide-area electron fields 218into the interior of the chamber 214. Similar to the conventionalfurnace 110 shown in FIG. 1, alloy bar 220 is introduced into chamber214 by a bar feeder 215. Molten alloy 226 is produced by impinging thewide-area electron field 218 of at least one wire-discharge ion plasmaelectron emitter 216 onto bar 220. The molten alloy 226 melted from bar220 drops into water-cooled copper hearth 224, is resident in the hearth224 for a certain dwell time, where it is heated, degassed, and refinedby one or more of the wide-area electron fields 218 produced by emitters216. The molten alloy 226 ultimately drops from hearth 224 into coppermold 230 and forms a molten pool 231. Molten pool 231 ultimately andprogressively solidifies in mold 230 to form ingot 232. At least one ofthe wide-area electron fields 218 preferably heats the molten alloywithin pool 231 in a manner advantageous to controlling thesolidification rate of the forming ingot 232.

As discussed above, the wire-discharge ion plasma electron emitters 216of furnace 210 are designed to generate a field or “flood” of energeticelectrons covering a wide area relative to the spot coverage of thesubstantially linear beam produced by electron beam guns used inconventional electron beam furnaces. The electron field emitters 216spread electrons over a wide area and impinge on the materials to bemelted and/or maintained in the molten state within the furnace 210.Because the electron field it produces will cover a wide area within thefurnace chamber, a wire-discharge ion plasma electron emitter willmaintain a more even temperature within the electron beam meltingfurnace relative to a conventional electron beam furnace, and alsodispenses with the need to raster a highly focused spot of electrons.Nevertheless, certain embodiments of the electron beam furnace accordingto the present disclosure may include components generating electricfields or other suitable components to steer the field of electronsgenerated by the one or more wire-discharge ion plasma electron emittersas desired. For example, in furnace 210 it may be desirable to rasterthe broad field produced by a wire-discharge ion plasma electron emitter216 from side-to-side to provide additional heat to edges of the hearth224. By flooding a relatively wide area with a field of energeticelectrons, rather than rastering a point source of electrons across thearea, the localized intense heating effect (e.g., power per unit area)associated with substantially linear electron beams that occurs whenusing conventional electron beam melting furnaces is significantlyreduced. This eliminates or at least significantly reduces the extent towhich relatively volatile alloying elements undesirably evaporate forthe reason that points of relatively extremely high temperature are notproduced. This, in turn, partially or wholly obviates the compositionalcontrol and contamination problems inherent in the conventional electronbeam furnace design.

As noted above, various embodiments of wire-discharge ion plasmaelectron emitters generally include one or more elongate wire anodesproducing positive ion plasma, wherein the plasma is impinged upon acathode to generate a field of secondary electrons that may beaccelerated to impinge on a target that is to be heated. A schematicrepresentation of one known design of a wire-discharge ion plasmaelectron emitter, previously used in other, unrelated, applications, isshown in FIG. 4. This emitter 310 includes an ionization or plasmaregion 314 in which a positive ion plasma is produced, and a cathoderegion 316 that includes cathode 318. The plasma region 314 is filledwith an ionizable gas at low pressure, and the gas is ionized in theplasma region to produce the cation-containing plasma. For example, theionization region 314 may be filled with helium gas at, for example,approximately 20 mTorr. A small diameter elongate wire anode 319 passesthrough a length of the plasma region 314. A positive voltage is appliedto wire anode 319 by power supply 322, and this initiates ionization ofthe helium gas into a plasma comprising helium cations and freeelectrons (the “primary” electrons). Once ionization of the helium gasis initiated, the plasma is sustained by applying a voltage to the thinwire anode 319. Positively charged helium ions within the plasma areextracted from the ionization chamber 314 through an extraction grid 326maintained at a high negative electrical potential and acceleratedthrough a high voltage gap into the cathode region 316, where thecations in the plasma impact high negative voltage cathode 318. Cathode318 may be, for example, a coated or uncoated metal or alloy.Impingement of helium ions on cathode 318 releases secondary electronsfrom cathode 318. The high voltage gap 328 accelerates the secondaryelectrons in a direction opposite the direction of movement of thehelium cations, through the extraction grid 326 and into the plasmaregion 314, and then through a thin metallic foil window 329 made ofmaterial relatively transparent to electrons. As noted above, dependingon the relative gas pressures within the electron emitter and thefurnace chamber, it may be possible to omit the foil window 329, inwhich case the electrons produced by the emitter would enter the furnacechamber directly.

The wire electrode 319 and the cathode 318 may be designed and arrangedto better promote movement of the positively charged helium ions to thecathode 318. Also, the cathode 318 and the extraction grid 326 may bedesigned and arranged to maximize secondary electron transmissionthrough the grid 326 and with a beam profile suitable for penetrationthrough foil window 329, if present. The wide-area field of energeticelectrons exiting the emitter 310 may be directed to impinge on a targetpositioned opposite foil window 329 and within the vacuum chamber of amelting furnace. Also, the window 329 may be sized to be as thin aspossible in order to maximize electron transmission from emitter 310. Analuminum-type or titanium-type foil having a thickness allowingsufficient electron transmission, while also maintaining a soft vacuumenvironment within the emitter 310, may be used as foil window 329, ifnecessary. Other suitably strong and acceptably electron transparentmaterials that may be used as a window in the apparatus, if present,will be known to those having ordinary skill. As discussed generallyherein, window 329 may be omitted if the pressure differences betweenthe interior of the emitter 310 and the vacuum chamber containing thetarget are not significant.

According to the present disclosure, one or more wire-discharge ionplasma electron emitters, such, for example, emitter 310, may beprovided to supply the energetic electrons into the vacuum chamber of anelectron beam melting furnace, as a substitute for electron beam gunsproducing a substantially linear electron beam. As shown in FIG. 5, onenon-limiting embodiment of an electron beam melting furnace according tothe present disclosure includes one or more wire-discharge ion plasmaelectron emitters 310 positioned adjacent vacuum chamber 330. Wide-areaelectron field 332 exits emitter 310 through film window 329 and floodsat least a region of the surface of the molten alloy 334 in hearth 336,thereby heating the alloy to maintain it in a molten state. Because theelectrons impinging on the alloy in hearth 336 are spread across arelatively wide area, the energy focused on the molten material in anyparticular localized region is not great enough to result in aproblematic level of volatilization of elements from the alloy, therebyreducing or obviating the alloy contamination and heterogeneity problemsinherent in the use of conventional electron beam melting furnaces. Asnoted above, film window 329 may be omitted if the operating pressuredifferential between the emitter 310 and the vacuum chamber 330 is notsignificant. Also, as noted above, the vacuum chamber 330 preferably isoperated at a pressure higher than is conventional in order to furtherreduce or eliminate undesirable elemental vaporization, and in such casethe need for a film window partitioning the electron emitter from thefurnace chamber will, again, depend on the particular pressuredifferential inherent in the design. Optionally, components 340 formagnetically steering the wide-area electron field are provided so as toallow further improved control of the melting process within the vacuumchamber 330.

Although FIG. 5 provides a simplified view of one embodiment of awire-discharge ion plasma electron melting furnace according to thepresent disclosure including a single electron emitter, it will beapparent to those of ordinary skill that actual or alternate embodimentsof such an apparatus may have multiple wire-discharge ion plasmaelectron emitters. It also will be apparent that one or morewire-discharge ion plasma electron emitters may be incorporated in suchan apparatus to: (1) melt raw materials introduced into the furnace, inthe form of, for example, an alloy bar or wire; (2) maintain moltenalloy resident in the furnace hearth at a temperature above the alloymelting temperature (and possibly degas and/or refine the molten alloy);and (3) maintain desired regions of the molten pool on the surface ofthe incrementally advancing cast ingot in a molten state, therebyinfluencing ingot solidification rate in a desired manner. Also, incertain embodiments, one or more wire-discharge ion plasma electronemitters may be used along with one or more electron beam guns producingconventional substantially linear electron beams.

FIGS. 6 and 7 provide additional details related to one possiblenon-limiting embodiment of a wire-discharge ion plasma electron emitterthat may be adapted for use as the source of energetic electrons in anembodiment of an electron beam melting furnace according to the presentdisclosure. FIG. 6 is a perspective view, partly in section, of thewire-discharge ion plasma electron emitter embodiment. FIG. 7 is aschematic diagram illustrating, in a simplified way, the operation ofemitter 510. Emitter 510 includes electrically grounded enclosure 513,which includes cathode region 511, ionization or plasma region 514, andelectron transmissive foil window 515. Elongate wire electrode 516extends through a length of ionization region 514. Foil window 515 iselectrically coupled to chamber 513 and thereby forms an anode thatoperates to accelerate electrons within chamber 513 therethrough to exitchamber 513 in the general direction of arrows “A”. Chamber 513 isfilled with helium gas at low pressure, such as 1-10 mtorr, and issupplied with the gas by gas supply 517. Gas supply 517 is connected toenclosure 513 by conduit 519, which passes through valve 521. A softvacuum environment is maintained in chamber 513 by pump 523, which isconnected to chamber 513 by conduit 526.

Cathode region 511 includes cathode 518, which in turn includes insert520 mounted on a lower surface thereof. The insert 520 may be composedof, for example, molybdenum, but may be composed of any material with asuitably high secondary electron emission coefficient. Cathode 518 issuitably uniformly spaced from the walls of enclosure 513 to preventPaschen breakdown. Cathode 518 is coupled to high voltage power supply522 by cable 525 which passes through insulator 526 and into resistor528. Power supply 522 supplies high negative potential, for example,200-300 KV, to cathode 518. Cathode 518 and insert 520 may be suitablycooled, such as by, for example, circulating oil or another suitablecooling fluid through conduits 524.

Ionization region 514 includes a plurality of thin metallic ribs whichare coupled both electrically and mechanically. Each rib 530 includes acentral cut-out region to allow wire electrode 516 to pass through theionization chamber 514. The sides of ribs 530 facing cathode 518 form anextraction grid 534. The opposed side of all or a portion of the ribs530 provide a support grid 536 for electron transmissive foil window515. Cooling channels 540 may be provided to circulate a cooling fluidthrough and in the vicinity of ribs 530 to allow for heat removal fromionization region 514. Electron transmissive foil window 515, which maybe composed of, for example, aluminum or titanium foil, is supported ongrid 534 and is sealed to enclosure 513 by an O-ring or other structuressufficient to maintain the high vacuum helium gas environment withinenclosure 513. In certain embodiments of emitter 510, a gas manifold 542is provided to cool foil window 515, such as with pressurized nitrogen.As discussed generally herein, window 515 may be omitted if the pressuredifferences between the interior of the chamber 513 of emitter 510 andthe chamber containing the target of the electron field are notsignificant.

An electrical control device 548 is connected to wire electrode 516through connector 549. On activation of control device 548, wireelectrode 516 is energized to a high positive potential, and heliumwithin ionization region 514 is ionized to produce a plasma includinghelium cations. Once the plasma is initiated in ionization region 514,cathode 518 is energized by power supply 522. Helium cations in theionization region 514 are electrically attracted to cathode 518 by theelectric field that extends from the cathode 518 into the plasma region514. The helium cations travel along the field lines, through theextraction grid 534, and into the cathode region 511. In the cathoderegion 511, the helium cations accelerate across the full potential ofthe electric field generated by the energized cathode 518 and forcefullyimpinge on the cathode 518 as a collimated beam of cations. Theimpacting cations free secondary electrons from the insert 520. Thesecondary electron field produced by the insert 520 is accelerated in adirection opposite the direction of travel of the helium cations, towardthe wire electrode 516, and through foil window 515, if present.

Means may be provided to monitor the actual gas pressure within thechamber 513 as changes in pressure may affect the density of the heliumion plasma and, in turn, the density of the secondary electron fieldgenerated at the cathode 518. An initial pressure may be set withinenclosure 513 by appropriately adjusting valve 521. Once thecation-containing plasma is initiated in plasma region 514, a voltagemonitor 550 may be provided to indirectly monitor the instantaneousquiescent pressure within the chamber 513. A rise in voltage isindicative of a lower chamber pressure. The output signal of the voltagemonitor 550 is used to control valve 521, through valve controller 552.The current supplied to wire electrode 516 by control device 548 also iscontrolled by the signal of voltage monitor 550. Utilizing the signalgenerated by voltage monitor 550 to control gas supply valve 521 andcontrol device 548 allows for a stable electron field output fromemitter 510.

The current generated by emitter 510 may be determined by the density ofthe cations impacting the cathode 518. The density of the cationsimpacting the cathode 518 may be controlled by adjusting the voltage onwire electrode 516 through control device 548. The energy of theelectrons emitted from the cathode 518 may be controlled by adjustingthe voltage on the cathode 518 through power supply 522. Both currentand energy of the emitted electrons can be independently controlled, andthe relationships between these parameters and the applied voltages arelinear, rendering control of the emitter 510 both efficient andeffective. In contrast, conventional thermo-ionic electron beam gunscannot be controlled in a corresponding linear manner when adjustingbeam parameters.

FIG. 8 is a schematic illustration of one embodiment of an electron beammelting furnace according to the present disclosure, wherein the furnace610 incorporates two wire-discharge ion plasma electron emitters 614,616 having a design as generally shown in FIGS. 6 and 7 and as discussedabove in connection with those figures. Furnace 610 includes vacuumchamber 620, material feeder 622, and casting or atomizing device 624.Current required for operation of emitters 614 and 616, as discussedabove, is fed to the emitters by power lines 626, and the interfacebetween emitters 614, 616 and the vacuum chamber 620 includes electrontransmissive foil windows 634, 636, which allow the electron fields 638generated by the emitters 614, 616 to enter the vacuum chamber 620. Thefoil windows 634, 636 may be omitted if the operating pressures withinthe emitters 614, 616 and the vacuum chamber are identical or do notsignificantly differ. Means 639 for magnetically steering electronfields 638 may be included within vacuum chamber 620 to provideadditional process control. A hearth 640, which may be, for example, acold hearth, is disposed in vacuum chamber 620. In operation,wire-discharge ion plasma electron emitters 614, 616 are energized andgenerate electron fields 618. An electrically conductive feed material644 is introduced into the vacuum chamber 620 by feeder 622, is meltedby electron field 638 emitted from emitter 614, and drops to hearth 640.Wide-area electron field 638 emitted by emitter 616 heats, degasses, andrefines the molten material 642 while resident in hearth 640. Moltenmaterial 642 advances along the hearth 640 and drops into casting oratomizing device 624 and is processed to a desired form.

Although the foregoing description has necessarily presented only alimited number of embodiments, those of ordinary skill in the relevantart will appreciate that various changes in the apparatus and methodsand other details of the examples that have been described andillustrated herein may be made by those skilled in the art, and all suchmodifications will remain within the principle and scope of the presentdisclosure as expressed herein and in the appended claims. For example,although the present disclosure has necessarily only presented a limitednumber of embodiments of electron beam melting furnaces according to thepresent disclosure, and also has necessarily only discussed a limitednumber of wire-discharge ion plasma electron emitter designs, it will beunderstood that the present disclosure and associated claims are not solimited. Those having ordinary skill will readily identify additionalwire-discharge ion plasma electron emitter designs and may design andbuild additional furnace designs along the lines and within the spiritof the necessarily limited number of embodiments discussed herein. It isunderstood, therefore, that the present invention is not limited to theparticular embodiments disclosed or incorporated herein, but is intendedto cover modifications that are within the principle and scope of theinvention, as defined by the claims. It will also be appreciated bythose skilled in the art that changes could be made to the embodimentsabove without departing from the broad inventive concept thereof.

We claim:
 1. An apparatus for melting an electrically conductivemetallic material, the apparatus comprising: a vacuum chamber; a hearthdisposed in said vacuum chamber; at least one wire-discharge ion plasmaelectron emitter disposed in or adjacent said vacuum chamber andpositioned to direct a wide-area field of electrons into said chamber,said wide-area field of electrons having sufficient energy to heat saidelectrically conductive metallic material to its melting temperature;and at least one of a mold and an atomizing apparatus in communicationwith said chamber and positioned to receive material from said hearth.2. The apparatus of claim 1, further comprising at least one feederadapted to introduce the electrically conductive material into thevacuum chamber in a position over at least a region of the hearth. 3.The apparatus of claim 2, wherein the feeder is selected from the groupconsisting of a bar feeder, a wire feeder, and particle feeder, and agranule feeder.
 4. The apparatus of claim 3, wherein the feeder and theat least one wire-discharge ion plasma electron emitter are disposed sothat the wide-area field of electrons emitted by the wire-discharge ionplasma electron emitter at least partially impinges on the electricallyconductive metallic material introduced into the chamber by the feeder.5. The apparatus of claim 1, wherein the hearth includes a moltenmaterial holding region, and wherein the hearth and the at least onewire-discharge ion plasma electron emitter are disposed so that thewide-area field of electrons emitted by the wire-discharge ion plasmaelectron emitter at least partially impinges on the molten materialholding region.
 6. The apparatus of claim 1, wherein the hearth isselected from the group consisting of a cold hearth and an autogenoushearth.
 7. The apparatus of claim 5, wherein the hearth is selected fromthe group consisting of a cold hearth and an autogenous hearth.
 8. Theapparatus of claim 1, wherein the mold is selected from the groupconsisting of a static mold, a withdrawal mold, and a continuous castingmold.
 9. The apparatus of claim 1, wherein the wire-discharge ion plasmaelectron emitter comprises a plasma region including a wire electrodeadapted to produce a positive ion plasma.
 10. The apparatus of claim 9,wherein the wire-discharge ion plasma electron emitter further comprisesa cathode region including a cathode electrically connected to a highvoltage power supply adapted to negatively charge said cathode, saidcathode positioned relative to said electrode so that positive ionsgenerated by said electrode are accelerated toward and impinge on saidcathode, liberating a wide-area field of electrons from said cathode.11. The apparatus of claim 10, wherein the wire-discharge ion plasmaelectron emitter further comprises an electron transmissive foil window,said foil window positioned within a wall of said chamber, therebyallowing electrons liberated from said cathode to enter said chamberthrough said foil window.
 12. The apparatus of claim 11, wherein saidfoil window comprises at least one of an electron transmissive titaniumfoil and an electron transmissive aluminum foil.
 13. The apparatus ofclaim 1, wherein said at least one wire-discharge ion plasma electronemitter opens into said vacuum chamber so that said wide-area field ofelectrons can pass directly from said at least one wire-discharge ionplasma electron emitter into said vacuum chamber without passing throughan electron transmissive window.
 14. The apparatus of claim 1, whereinsaid apparatus does not include an electron transmissive windowintermediate said at least one wire-discharge ion plasma electronemitter and said vacuum chamber.
 15. The apparatus of claim 9, whereinsaid plasma region opens into said vacuum chamber so that said wide-areafield of electrons can pass from said plasma region into said vacuumchamber without passing through an electron transmissive window.
 16. Theapparatus of claim 9, wherein said apparatus does not include anelectron transmissive window intermediate said plasma region and saidvacuum chamber.
 17. The apparatus of claim 9, wherein said plasma regionis fluidly communicates with said vacuum chamber.
 18. The apparatus ofclaim 10, wherein said cathode comprises an insert having a high meltingtemperature and a low work function.
 19. The apparatus of claim 18,wherein said insert comprises molybdenum.
 20. The apparatus of claim 10,wherein said high voltage power supply powers said cathode to a negativevoltage greater than 20,000 volts.
 21. The apparatus of claim 9, whereinthe electrode comprises a wire positioned along a length of said plasmachamber.
 22. The apparatus of claim 1, wherein the apparatus is anelectron beam cold hearth melting furnace and the electricallyconductive metallic material is at least one of a material selected fromtitanium, titanium alloys, tungsten, niobium, tantalum, platinum,palladium, zirconium, iridium, nickel, nickel base alloys, iron, ironbase alloys, cobalt, and cobalt base alloys.
 23. The apparatus of claim1, wherein the apparatus is an electron beam cold hearth melting furnacecomprising: a vacuum chamber; a hearth disposed in said vacuum chamber,said hearth including a molten material holding region; at least onewire-discharge ion plasma electron emitter disposed in or adjacent saidvacuum chamber, wherein the hearth and the at least one wire-dischargeion plasma electron emitter are disposed so that a wide-area electronfield emitted by the wire-discharge ion plasma electron emitter at leastpartially impinges on the molten material holding region; a withdrawalmold in communication with said chamber and positioned to receivematerial from said hearth; and at least one feeder adapted to introducethe electrically conductive material into the vacuum chamber in aposition over at least a region of the hearth.
 24. The apparatus ofclaim 23, wherein the wire-discharge ion plasma electron emittercomprises a plasma region including an electrode adapted to produce apositive ion plasma.
 25. The apparatus of claim 24, wherein thewire-discharge ion plasma electron emitter further comprises a cathoderegion including a cathode electrically connected to a high voltagepower supply adapted to negatively charge said cathode, said cathodepositioned relative to said electrode so that positive ions generated bysaid electrode are accelerated toward and impinge on said cathode,liberating a wide-area field of electrons from said cathode.
 26. Theapparatus of claim 25, wherein the wire-discharge ion plasma electronemitter further comprises an electron transmissive foil window, saidfoil window positioned within a wall of said chamber, thereby allowingelectrons liberated from said cathode to enter said chamber,
 27. Theapparatus of claim 26, wherein said high voltage power supply powerssaid cathode to a negative voltage greater than 20,000 volts.